1. Field of the Invention
The present invention relates to a slip control system of a motor vehicle, in particular to a slip control system in which slips in driven wheels are controlled by operations of braking force control and engine output control.
2. Description of the Related Art
There is known a conventional slip control system for a motor vehicle which prevents a decrease in acceleration ability caused by the slip which is caused by an excessive driving torque of the driving wheel in an accelerating operation and the like. In order to do so, such slip control system detects a slip value of the driven wheel and controls an engine output and braking force so that the slip value of the driven wheel becomes less than a target slip value which is provided based on friction coefficient of the road surface. The engine output is decreased by the engine output control and the braking force is increased by the braking force control. In the engine output control, a throttle opening in a throttle valve of an engine is controlled.
On the other hand, since a responsibility of the braking force control is larger than that of the engine output control in the slip control system, a brake device absorbs large energy so as to overcome the excessive driving torque of the engine. As a result, the frequent brake force control under high load conditions causes a decrease in the liability of the brake device and vibration of the motor body. Therefore, it has been demanded that amount and time of the engine torque to be absorbed by the brake device should be as small as possible.
Japanese Patent Laid-Open Publication No. 3-109161 discloses a slip control system for a motor vehicle which can solve the above-mentioned problems. Such slip control system detects the sum of the braking pressures in both right and left driving wheels by estimating braking pressures in both right and left driven wheels, and decreases the engine output by amount thereof corresponding to the sum of the braking pressures in the driven wheels so that the load in the brake device is decreased.
However, if the engine output is decreased by the amount thereof corresponding to the sum of the braking pressure in the both right and left driven wheels as shown in the above Patent Publication when the slips in the driven wheels are increased rapidly, the throttle valve is closed too much which causes some problems such as the engine being stalled when the slip is recovered and acceleration being delayed.
On the other hand, in a cold district, the vehicle often runs on so called split road whose friction coefficients against both right and left driven wheels are different each other. In such cases, if the engine output is decreased by the amount thereof corresponding to the sum of the braking pressure in the both driven wheels as shown in the above Patent Publication, even though one driven wheel has enough grip force against the road surface, driving torques in both if one and other driven wheels are decreased. As a result, starting ability and acceleration ability is decreased.